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detect VidHD in passive mode

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4am 2019-10-05 16:40:29 -04:00
parent 686539f8b3
commit 352441f2b9
1 changed files with 36 additions and 2 deletions
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38
src/hw.vidhd.a
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@ -16,7 +16,7 @@
; A/Y clobbered
;------------------------------------------------------------------------------
HasVidHDCard
ldx #$c7
ldx #$C7
@slotLoop
stx @byteLoop+2
ldy #$02
@ -26,12 +26,46 @@ HasVidHDCard
bne @nextSlot
dey
bpl @byteLoop
@found
sec ; found
rts
@nextSlot
dex
cpx #$c0
cpx #$C0
bne @slotLoop
; check for a slot where the Cx00 page is all zeroes
; and the devsel bytes are also all zeroes, which
; indicates that VidHD is probably present but is in
; passive mode because some other card is hogging DMA
ldx #$C7
@passiveSlotLoop
stx @passiveByteLoop+2
ldy #0
@passiveByteLoop
lda $FD00, y ; SMC (high byte)
bne @passiveNextSlot
iny
bpl @passiveByteLoop
txa
and #$0F ; A = $01..$07
asl
asl
asl
asl ; A = $10..$70
ora #$80 ; A = $90..$F0
sta @devselLoop+1
ldy #$0F
@devselLoop
lda $C0FD, y ; SMC (low byte)
bne @passiveNextSlot
dey
bpl @devselLoop
bmi @found ; always branches
@passiveNextSlot
dex
cpx #$C0
bne @passiveSlotLoop
clc ; not found
rts
@kVidHDMagicBytes